To prevent global warming, the reduction of the amount of a CO2 gas exhausted from automobiles is strongly demanded. To reduce the amount of a CO2 gas exhausted, it is mainly necessary to improve the fuel efficiency of automobiles. Technologies for improving fuel efficiency include fuel direct injection systems, increase in compression ratios, the reduction (downsizing) of engine weights and sizes by supercharging, increase in the boost pressure of turbochargers, etc. With these technologies introduced, fuel combustion tends to occur at higher temperatures and higher pressure in engines, so that the temperatures of exhaust gases discharged from engine combustion chambers to exhaust members such as exhaust manifolds, catalyst cases, etc. are elevated to nearly 1000° C. Exhaust members exposed to such high-temperature exhaust gases are required to have excellent heat resistance characteristics (oxidation resistance, thermal cracking resistance, thermal deformation resistance). To exhaust manifolds, etc. among the exhaust members, oxidation resistance and thermal cracking resistance are particularly important.
Conventionally used for exhaust members such as exhaust manifolds, etc. used under severe conditions at high temperatures are heat-resistant cast iron such as high-Si, spheroidal graphite cast iron and Ni-Resist cast iron (austenitic cast Ni—Cr iron), heat-resistant, ferritic cast steel, heat-resistant, austenitic cast steel, etc. Ferritic, spheroidal graphite cast iron containing 4% Si and 0.5% Mo exhibits relatively good heat resistance up to about 800° C., but is poor in durability at temperatures higher than 800° C. Heat-resistant cast iron such as Ni-Resist cast iron and heat-resistant, austenitic cast steel, which contain large amounts of rare metals such as Ni, Cr, Co, etc., have satisfactory oxidation resistance and thermal cracking resistance at temperatures of 800° C. or higher. However, the Ni-Resist cast iron is expensive because of a large Ni content, and has poor thermal cracking resistance because it has a large coefficient of linear expansion due to an austenitic matrix structure, and because its microstructure contains graphite acting as the starting points of fracture. The heat-resistant, austenitic cast steel has insufficient thermal cracking resistance at about 900° C. because of a large coefficient of linear expansion, though not containing graphite acting as the starting points of fracture. In addition, it is expensive because it contains large amounts of rare metals, and unstable in material supply affected by world economic conditions.
From the aspect of economic feasibility, stable material supply and efficient use of resources, heat-resistant cast steels for exhaust members desirably have necessary heat resistance with the amounts of rare metals minimized. Thus provided are inexpensive, high-performance exhaust members, which enable the application of fuel-efficiency-improving technologies to inexpensive popular cars, contributing to reducing the amount of a CO2 gas exhausted. To reduce the amounts of rare metals contained as much as possible, the matrix structures of alloys are advantageously ferrite rather than austenite. In addition, because the heat-resistant, ferritic cast steel has a smaller coefficient of linear expansion than that of the heat-resistant, austenitic cast steel, the former has better thermal cracking resistance because of smaller thermal stress generated at the start and acceleration of engines.
The heat-resistant, ferritic cast steel has poor toughness at room temperature, because it contains a large amount of Cr for oxidation resistance. Exhaust members are subject to mechanical vibration and shock in their production process, their assembling process to engines, etc. Accordingly, heat-resistant, ferritic cast steels used for exhaust members should have sufficient room-temperature toughness to avoid cracking and fracture by mechanical vibration and shock.
JP 2007-254885 A discloses a thin casting part made of Fe-based, ferritic, cast stainless steel comprising 0.10-0.50% by mass of C, 1.00-4.00% by mass of Si, 0.10-3.00% by mass of Mn, 8.0-30.0% by mass of Cr, and 0.1-5.0% by mass of Nb and/or V, which has thin portions having thickness of 1-5 mm, a ferrite phase in the structure of thin portions having an average crystal grain size of 50-400 μm to exhibit improved high-temperature strength. Because thin portions of 5 mm or less in this thin casting part are rapidly cooled after casting, the ferrite phase has a small average crystal grain size, resulting in high high-temperature yield strength, high tensile strength and large fracture elongation.
However, exhaust members have cylinder-head-mounting flanges, heat-insulation-plate-mounting bosses, bolt-fastening portions, etc., which are as thick as 5 mm or more, and so low in cooling speeds. The cooling speed is also low in portions near risers for preventing shrinkage cavities, and portions formed by adjacent cavities in a sand mold which tend to be overheated, even though they are as thin as 5 mm or less. Such low-cooling-speed portions have large average crystal grain sizes, resulting in low room-temperature toughness. However, JP 2007-254885 A fails to disclose means for suppressing toughness decrease. Also, the heat-resistant, ferritic cast steel of JP 2007-254885 A has improved melt flowability, which is obtained by lowering its melting point by containing a large amount of Si, and high-temperature strength, oxidation resistance, carburizing resistance and machinability, but it has low room-temperature toughness because it contains Si in as large an amount as 1.00-4.00% by mass (about 2% or more in Examples), Si being dissolved in a ferritic matrix structure. The average crystal grain size should be made smaller even in other portions than thin portions to obtain high room-temperature toughness, and the solid solution of alloying elements in the matrix structure should be minimized to avoid embrittlement. However, JP 2007-254885 A does not achieve these objectives.
JP 7-197209 A discloses heat-resistant, ferritic cast steel having a composition comprising by weight 0.15-1.20% of C, 0.05-0.45% of C—Nb/8, 2% or less of Si, 2% or less of Mn, 16.0-25.0% of Cr, 1.0-5.0% of W and/or Mo, 0.40-6.0% of Nb, 0.1-2.0% of Ni, and 0.01-0.15% of N, the balance being Fe and inevitable impurities, and having an α′ phase (α+carbide) transformed from a γ phase (austenite phase), in addition to a usual a phase (a ferrite phase), the area ratio of the α′ phase [α′/(α+α′)] being 20-70% to improve castability. Because this heat-resistant, ferritic cast steel contains C (austenitizing element) in an amount more than necessary for forming NbC, C dissolved in the matrix structure forms a γ phase when solidified. The γ phase is transformed to an α′ phase in a cooling process, thereby improving ductility and oxidation resistance. Accordingly, this heat-resistant, ferritic cast steel is suitable for exhaust members used at 900° C. or higher.
However, the transformation of the γ phase to the α′ phase does not proceed sufficiently in an as-cast state, and the γ phase is transformed to a martensite phase. Because the martensite phase has high hardness, it extremely deteriorates room-temperature toughness and machinability. To secure high toughness and machinability, a heat treatment for erasing the martensite phase and precipitating the α′ phase is needed. Because a heat treatment generally increases production costs, it damages the economic advantage of the heat-resistant, ferritic cast steel containing small amounts of rare metals.
JP 11-61343 A discloses heat-resistant, ferritic cast steel having a composition comprising by weight 0.05-1.00% of C, 2% or less of Si, 2% or less of Mn, 16.0-25.0% of Cr, 4.0-20.0% of Nb, 1.0-5.0% of W and/or Mo, 0.1-2.0% of Ni, and 0.01-0.15% of N, the balance being Fe and inevitable impurities, and having a Laves phase (Fe2 M) in addition to a usual a phase, thereby having high-temperature strength, particularly excellent creep rupture strength. Though this heat-resistant, ferritic cast steel has improved high-temperature strength, particularly creep rupture strength because of a Laves phase by a combination of Nb, W, Mo, Ni and N, its room-temperature toughness is not necessarily sufficient because it contains large amounts of alloying elements.